C h a p t e r 1 9
Disorders of Cardiac Function
477
of the great arteries). Intravenous infusion of pros-
taglandin E
1
(PGE
1
) has proved extremely effective in
maintaining ductal patency or reopening the ductus in
newborns. Today, this therapy is routinely administered
to newborns with suspected congenital heart defects
until they can be transported to a specialized center
where a diagnosis can be confirmed.
80
Atrial Septal Defects.
In atrial septal defects, an open-
ing in the atrial septum persists as a result of improper
septal formation
81
(see Fig. 19-21A). Partitioning of the
atria takes place during the 4th and 5th weeks of devel-
opment and occurs in two stages, beginning with the
formation of a thin, crescent-shaped membrane called
the
septum primum
followed by the development of a
second membrane called the
septum secundum.
As the
septum secundum develops, it gradually overlaps an
opening in the upper part of the septum primum, form-
ing an oval opening with a flap-type valve called the
foramen ovale
(Fig. 19-22). The foramen ovale, which
closes shortly after birth, allows blood from the umbili-
cal vein to pass directly into the left heart, bypassing the
lungs.
Atrial septal defects may be single or multiple and
vary from a small, asymptomatic opening to a large,
symptomatic opening. The type of defect is determined
by its position and may include an abnormal opening in
the septum primum (ostium primum defects), the sep-
tum secundum (ostium secundum defects), or a patent
foramen ovale. An ostium secundum atrial septal defect
in the region of the foramen ovale is the most common
defect. The defect may be single or multiple (fenestrated
atrial septum). Most atrial septal defects are small and
young children with these defects are often asymptom-
atic, with the defects discovered inadvertently during
a routine physical examination at a few years of age.
81
In the case of an isolated septal defect large enough to
allow shunting, the flow of blood usually is from the left
side to the right side of the heart because of the more
compliant right ventricle and because the pulmonary
vascular resistance is lower than the systemic vascular
resistance. This produces right ventricular volume over-
load and increased pulmonary blood flow. In most cases
there is a moderate shunt resulting in dilation of the
right heart chambers and overperfusion of the pulmo-
nary circulation.
Children with undiagnosed atrial defects are at risk
for pulmonary vascular disease, although this is a rare
occurrence before 20 years of age. Rarely, infants with
a large shunt may develop congestive heart failure and
failure to thrive, prompting early intervention to close
the defect.
81
Adolescents and young adults may experi-
ence atrial flutter or atrial fibrillation and palpitations
because of atrial dilatation. Larger symptomatic defects
are usually treated surgically or by transcatheter device
closure. Smaller defects may be observed for spontane-
ous closure in the young child.
Ventricular Septal Defects.
A ventricular septal defect
is an opening in the ventricular septum that results from
an incomplete separation of the ventricles during early
fetal development (see Fig. 19-21B). These defects may
be single or multiple and may occur in any position
along the ventricular septum. Ventricular septal defects
are the most common form of congenital heart defect,
accounting for 25% to 30% of congenital heart disor-
ders.
82
A ventricular septal defect may be the only car-
diac defect, or it may occur in association with multiple
cardiac anomalies.
The ventricular septum originates from two sources:
the interventricular groove of the folded tubular heart
that gives rise to the muscular part of the septum, and
the endocardial cushions that extend to form the mem-
branous portion of the septum. The upper membranous
portion of the septum is the last area to close, typically
by the 7th week of gestation, and it is here that most
defects occur.
Depending on the size of the opening and the pulmo-
nary vascular resistance, the signs and symptoms of a
ventricular septal defect may range from an asymptom-
atic murmur to congestive heart failure.
82
If the defect
is small, it allows a small shunt and small increases in
pulmonary blood flow. These defects produce few symp-
toms, and approximately one third close spontaneously.
With medium-sized defects, a larger shunt occurs, pro-
ducing a larger increase in pulmonary blood flow. Most
of the children with such defects are asymptomatic and
have a low risk for development of pulmonary vascular
disease.
In children with large nonrestrictive defects, right and
left ventricular pressure is equalized and the degree of
shunting is determined by the ratio of pulmonary to sys-
temic vascular resistance. Pulmonary vascular resistance
normally falls rapidly after birth, owing to the onset of
ventilation and subsequent release of hypoxic pulmo-
nary vasoconstriction. This process is often delayed in
infants with large ventricular septal defects. As a result,
the pulmonary vascular resistance remains somewhat
Septum secundum
Septum primum
Foramen
ovale
Endocardial
cushions
Right
atrioventricular
canal
Left
atrioventricular
canal
Interventricular
septum
FIGURE 19-22.
Development of the endocardial cushions,
right and left atrioventricular canals, interventricular septum,
and septum primum and septum secundum of the foramen
ovale. Note that blood from the right atrium flows through the
foramen ovale to the left atrium.
